The heart is a muscular organ comprising multiple chambers that operate in concert to circulate blood throughout the body's circulatory system. As shown in FIG. 1, the heart 100 includes a right-side portion or pump 102 and a left-side portion or pump 104. The right-side portion 102 includes a right atrium 106 and a right ventricle 108. Similarly, the left-side portion 104 includes a left atrium 110 and a left ventricle 112 separated by an interventricular septum 105. Oxygen-depleted blood returning to the heart 100 from the body collects in the right atrium 106. When the right atrium 106 fills, the oxygen-depleted blood passes into the right ventricle 108 where it can be pumped to the lungs (not shown) via the pulmonary arteries 117.
Within the lungs, waste products such as carbon dioxide are removed from the blood and expelled from the body and oxygen is transferred to the blood. Oxygen-rich blood returning to the heart 100 from the lungs via the pulmonary veins (not shown) collects in the left atrium 110. The circuit between the right-side portion 102, the lungs, and the left atrium 110 is generally referred to as the pulmonary circulation. After the left atrium 110 fills, the oxygen-rich blood passes into the left ventricle 112 where it can be pumped throughout the entire body. In so doing, the heart 100 is able to supply oxygen to the body and facilitate the removal of waste products from the body.
To circulate blood throughout the body's circulatory system as described above, a beating heart performs a cardiac cycle that includes a systolic phase and a diastolic phase. During the systolic phase, or systole, the ventricular muscle cells of the right and left ventricles 108 and 112 contract to pump blood through the pulmonary circulation and throughout the body, respectively. Conversely, during the diastolic phase, or diastole, the ventricular muscle cells of the right and left ventricles 108 and 112 relax, during which the right and left atriums 106 and 110 contract to force blood into the right and left ventricles 108 and 112, respectively. Typically, the cardiac cycle occurs at a frequency between 60 and 100 cycles per minute and can vary depending on physical exertion and/or emotional stimuli, such as pain or anger.
Normally, the muscular walls of each chamber of the heart 100 contract synchronously in a precise sequence to efficiently circulate the blood as described above. In particular, both the right and left atriums 106 and 110 contract and relax synchronously. Shortly after the atrial contractions, both the right and left ventricles 108 and 112 contract and relax synchronously.
The synchronous contraction of the heart can be disrupted in an individual who suffers from congestive heart failure. For example, a patient suffering from congestive heart failure or other similar heart conditions can periodically decompensate to a point at which the individual's heart cannot pump sufficient blood to sustain even mild physical activity. In such a situation, the individual is typically hospitalized and treated with a regimen of drugs to increase hemodynamic performance and reduce cardiac demand. This drug regimen is typically applied for several days to several weeks until hemodynamic performance is improved sufficiently.
Decreased cardiac efficiency can also be treated chronically through implantation of a cardiac rhythm management system including a cardiac resynchronization therapy device. A cardiac resynchronization therapy device can apply cardiac resynchronization therapy, which is a process involving the application of an electrical stimulus to the left ventricle or both the left and right ventricles after pacing has been detected in the atria. This electrical stimulus forces septum 105 and free wall 134 of heart 100 to contract at approximately the same time, thereby resynchronizing left ventricular contraction and improving hemodynamic performance. See, for example, U.S. Pat. Nos. 6,480,742 and 6,622,040, both of which describe cardiac resynchronization devices and are hereby incorporated by reference in their entireties.
There is a need for additional systems and methods that can improve an individual's hemodynamic performance after a decompensation event.